Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates generally to power supplies for computer systems. More specifically, the invention relates to switching power supplies for computer systems. More specifically still, the present invention relates to reducing the harmonic distortion generated by a switching power supply.
2. Background of the Invention
As computer system technology advances, specifically as manufacturing techniques related to microprocessors or central processing units (CPUs) advances, more and more transistors and related functionality are placed on a single die of a CPU. As more transistors are placed closer together on semiconductor substrates, less insulation material (in the form of oxide layers) exists between each transistor. Accordingly, CPU operating voltages are lowered to protect against electrical breakdown between transistors. However, adding transistors to a CPU increases the amount of electrical current the CPU requires. Thus, while the operating voltage for CPUs generally has been dropping as technology advances, required operating currents have steadily risen. Every computer system has a power supply that converts the 120 Volt alternating current (AC) found in a standard wall receptacle to suitable direct current (DC) voltages. This conversion from AC to DC is typically done by a switching power supply. A switching power supply should be capable of supplying current swings having transient response in the range of 100 Amps per micro-second. Thus, there are increasing demands on the capabilities of switching power supplies with each advance in CPU technology.
FIG. 1 shows an exemplary partial electrical schematic of a single phase buck-type switching power supply. The circuit shown in FIG. 1 is said to have only a single phase because it has only one switch and inductor combination. If there were several of these switch and inductor combinations present, the power supply would be considered a multi-phase switching power supply. Buck-type switching power supplies are designed to provide lower direct current (DC) voltages while supplying the current demand of a load (e.g., CPU). This ability to provide reduced DC voltages is accomplished by xe2x80x9cchoppingxe2x80x9d the supply voltage (i.e. turning on and off at a particular frequency )via switch 1 and then averaging, by means of an inductor/capacitor circuit 2, the chopped voltage to produce DC voltage at the desired level.
In the early days of microprocessor technology, a computer system switching power supply may have had only a single phase, as explained above, inasmuch as the processor voltage and amperage requirements were such that a single phase switching power supply was capable of producing the desired voltage with the desired current. While a single phase switching power supply may be capable of meeting average voltage and current requirements, a single phase alone may not be capable of meeting higher transient requirements of modem CPUs. Another consideration in switching power supply design, especially as related to power supplies mounted on a motherboard, is the amount of space required to implement such a supply. If a single switching phase is used, the inductor and capacitor in the averaging portion of the circuit may need to be excessively large occupying too much space on the motherboard.
Increasing the number of phases in a switching power supply permits the capacitor and inductor in each phase to be smaller, as is well understood in the art. Thus, in response to demands such as space limitations and transient current response, manufacturers generally increase the number of phases in switching power supplies. However, there are various problems associated with having multiple phases in a switching power supply. For example, xe2x80x9charmonicsxe2x80x9d generated on the supply to the switching power supply. A harmonic, in the context of a switching power supply, is undesirable high frequency noise generated on the input signal or supply line of the switching power supply. If such harmonics are generated, it is possible that they may interfere with other components of the computer system causing improper operation.
Switching power supplies create harmonics in the process of chopping input voltages to create the desired output voltage level. Chopping of the input voltage creates significant swings in current supplied to the switching power supply. As an example, consider a buck-type switching power supply having three switching phases, each phase substantially equivalent to that shown in FIG. 1. Each phase of the switching power supply has its conducting switch 1 operable (opening and closing) at the same frequency; however, each switch activates at a different phase. FIG. 2A shows an exemplary timing diagram of the switching signals applied to the control switches of a three-phase buck-type switching power supply. The duty cycle of the switching signal applied to each control switch, in this example 50% duty cycle, is sufficiently large that two phases of the three-phase switching power supply conduct simultaneously. FIG. 2A indicates these time periods by shading. As each of these phases of the switching power supply become active, the phase draws current from the supply voltage. When two phases conduct, the current demand on the supply to the switching power supply increases over the requirement for a single phase. Thus, there are times in this example when two phases draw current from the input source simultaneously, and there are times when only a single phase draws current from the current source. A graph as function of time of the AC current demand on an input signal line for the exemplary three phase system with 50% duty cycle is shown in FIG. 3A. The higher demand times shown in FIG. 3A, in this example approximately 10 Amps AC, are the periods of time when two phases of the switching power supply are active. In an exemplary three phase system with a desired output voltage of 1.5 volts, a desired output current of 50 Amps DC, a switching frequency on a per phase basis is 200 kHz, and an inductor of each phase of 2 micro-henries, the Root Mean Square (RMS) current exemplified in FIG. 3A is 8.0 Amps AC.
FIG. 2B shows switch enable signals applied to a three-phase buck-type switching power supply. The duty cycle of the switch enable signals FIG. 2B is 25% to exemplify the current distribution on the input signal when there are gaps between conducting periods of the control switches. As indicated by the shaded portions in FIG. 2B, a certain amount of time exists between conduction periods of each phase. As each phase conducts, this places a demand for current on the input to the switching power supply. FIG. 3B exemplifies a current demand on an input signal line for a three-phase switching power supply where the switch enable signals have a cycle of 25%. For such a system, the RMS current demand at the input of the three-phase switching power supply would be 8.0 Amps AC.
A single or multiple-phase switching power supply creates RMS currents on the input which create harmonics. These harmonics in some circumstances may propagate back to other devices within the computer system causing errors if they are not properly filtered. Filtering however is not an entirely acceptable solution because these filters require valuable motherboard space.
Thus, it would be desirable to have a system and/or method for reducing the RMS currents, harmonics, generated on an input to a switching power supply to alleviate the problems associated with harmonics noted above.
The problems noted above are solved in large part by a method and apparatus whereby the input voltage to a buck-type switching power supply is adjusted such that there is little if any overlap in conduction periods of the various phases of the switching power supply, and conversely, there is little if any dead time between conduction periods. In this way, the amperage demand on the input of the switching power supply is relatively constant which reduces the RMS current and correspondingly reduces harmonics generated on the input signal line to the switching power supply.